The CDMA technique is used in third generation systems such as the Universal Mobile Telecommunication System (UMTS).
As a general rule, a mobile radio network includes a set of base stations and base station controllers (see FIG. 1). In the UMTS, this network is called the UMTS Terrestrial Radio Access Network (UTRAN), a base station is called a Node B, and the base station controllers are called Radio Network Controllers (RNC).
The UTRAN communicates with a mobile station (also called a User Equipment (UE)), via a Uu interface and with a Core Network (CN) via an lu interface.
As shown in FIG. 1, the radio network controllers are connected:                to a Node B via a lub interface,        to each other via a lur interface, and        to the Core Network CN via an lu interface.        
The radio network controller that controls a given Node B is called its Controlling Radio Network Controller (CRNC) and is connected to the Node B via the lub interface. The functions of the CRNC are load control and radio resource control and allocation for each Node B that it controls.
A Serving Radio Network Controller (SRNC) for a given call relating to a given User Equipment UE is connected to the Core Network CN via the lu interface. The SRNC has a control function for the call concerned, including functions of adding or withdrawing radio links (for the macrodiversity transmission technique) and monitoring parameters that may change during a call, such as bit rate, power, spreading factor, etc.
In CDMA systems, the radio interface capacity limitations are fundamentally different from those in systems using other multiple access techniques, such as the Time Division Multiple Access (TDMA) technique. The TDMA technique is used in second generation systems such as the Global System for Mobile communications (GSM). In CDMA systems, all users share the same frequency resource at all times. The capacity of these systems, which are known as soft limited systems, is therefore limited by interference.
CDMA systems use algorithms such as load control algorithms for preventing, detecting and where appropriate correcting overloads, to prevent quality being degraded, and call admission control algorithms for deciding (as a function of various parameters such as the service required for the call, etc.) if the capacity of a cell that it is not being used at a given time is sufficient for a new call to be accepted in that cell. In the remainder of this description these algorithms are referred to generically as load control algorithms.
As a general rule they apply only radio criteria and are implemented in the CRNC, which has no information on the processing capacity of any Node B that it controls. This being so, it can happen that a new call is accepted by the CRNC but is rejected in the end for lack of processing resources in the Node B, which causes unnecessarily additional processing in the CRNC and additional exchanges of signaling between the CRNC and the Node B.
It would naturally be possible to avoid these problems by providing a Node B with sufficient processing resources to cover all situations, including situations of maximum capacity (corresponding to a very low level of interference). However, this would make the base stations costly, and they would have a better specification than would be required most of the time. Also, in the case of progressive introduction of the services offered by these systems, the processing capacity of the base stations may be limited when the systems begin to come into service and progressively increased thereafter.
It would therefore be desirable to take account of base station (Node B) processing capacity in controlling the load in any such system.
FIGS. 2 and 3 respectively indicate the main send and receive processing employed in a base station such as a UMTS Node B. This processing is well known to the person skilled in the art and does not need to be described in detail here.
FIG. 2 shows a sender 1 including:                channel coder means 2,        spreader means 3, and        radio frequency sender means 4.        
Channel coding uses techniques such as error corrector coding and interleaving to protect against transmission errors. This is known in the art.
The coding (for example error corrector coding) introduces redundancy into the information transmitted. The coding rate is defined as the ratio of the number of information bits that need to be transmitted to the number of bits actually transmitted or coded. Different levels of quality of service can be obtained using different types of error corrector code. For example, in the UMTS, a first type of error corrector code (a turbo code) is used for a first type of traffic (such as high bit rate data) and a second type of error corrector code (a convolutional code) is used for a second type of traffic (such as data at a lower bit rate or voice).
Channel coding usually also includes bit rate adaptation to adapt the bit rate to be transmitted to the bit rate available for transmission. Bit rate adaptation can include techniques such as repetition and/or puncturing, and the bit rate adaptation rate is then defined as the repetition and/or puncturing rate.
The raw bit rate is defined as the bit rate actually transmitted at the radio interface. The net bit rate is the bit rate obtained after deducting from the raw bit rate everything that is not useful to the user, in particular the redundancy introduced by coding.
Spreading uses spectrum spreading techniques known to the person skilled in the art. The length of the spreading code used is called the spreading factor.
In a system such as the UMTS the net bit rate (referred to hereinafter simply as the bit rate) can vary during the same call and the spreading factor can vary as a function of the bit rate to be transmitted.
FIG. 3 shows a receiver 5 including:                radio frequency receiver means 6, and        received data estimator means 7 including despreader means 8 and channel decoder means 9.        
The corresponding processing is also well known to the person skilled in the art and therefore does not need to be described in detail here.
FIG. 3 shows an example of the processing carried out in the despreader means 8. In this instance the processing corresponds to that carried out in a Rake receiver to improve the quality of the received data estimate by exploiting multipath phenomena, i.e. propagation of the same source signal along multiple paths, due in particular to multiple reflections from features of the environment. In CDMA systems, unlike TDMA systems, multipath propagation can be exploited to improve the quality of the received data estimate.
A Rake receiver has a set of L fingers 101 to 10L and combiner means 11 for combining signals from the various fingers. Each finger despreads the signal received over one of the paths, which are determined by estimator means 12 for estimating the impulse response of the transmission channel. To optimize the quality of the received data estimate the combiner means 11 combine the despread signals corresponding to the various paths.
The receiving technique using a Rake receiver is also used in conjunction with the macrodiversity transmission technique, whereby the same source signal is transmitted simultaneously to the same mobile station by a plurality of base stations. The macrodiversity transmission technique not only improves receive performance, by using a Rake receiver, but also minimizes the risk of calls being lost during handover. This technique is known as soft handover, as opposed to the hard handover technique whereby a mobile station is connected to only one base station at any given time.
The received data estimator means can also use various techniques to reduce interference, for example the multi-user detection technique.
It is also possible to use a plurality of receive antennas. The received data estimator means then further include combiner means for combining signals obtained via the various receive antennas to optimize the quality of the received data estimate.
Channel decoding includes functions such as de-interleaving and error corrector decoding. Error corrector decoding is generally much more complex than error corrector coding and can use techniques such as maximum likelihood decoding, for example. A Viterbi algorithm can be used for convolutional codes, for example.
To be able to process several users at the same time, a base station or Node B includes a set of senders and receivers like the sender and the receiver referred to above. A base station or Node B therefore requires a high processing capacity, in particular on the receiving side, for estimating received data.
As previously indicated, it is therefore desirable to take account of the processing capacity of a base station for implementing load control in a system such as the UMTS, for example.
In the case of the UMTS, the document 3G TS 25.433 published by the 3rd Generation Partnership Project (3GPP) requires that the Node B signal to the CRNC its global processing capacity (capacity credit) and the quantity of that capacity credit (allocation cost) necessary for allocating a physical channel, for each value of the spreading factor (SF) available in the system. The set of allocation costs for the available values of the spreading factor is called the capacity consumption law. The combination of a capacity credit and a consumption law is called a resource model. This kind of information is signaled by a Node B to the CRNC each time that the processing capacity of the Node B changes, using a Resource Status Indication Message, or in response to a request from the CRNC, using an Audit Response message.
A first prior art patent application (French Patent Application No. 00/105358 filed Aug. 10, 2000 by the applicant), points out that a solution of this kind is not suitable for taking account of limitations in the processing capacity of a Node B, for the following reasons in particular:                The channel decoding processing depends on the net bit rate rather than the raw bit rate or the spreading factor. For example, the net bit rate can take different values depending on the coding rate and the bit rate adaptation rate, and can typically vary from 5 kbps to 15 kbps with a spreading factor of 128 (and thus a raw bit rate of 30 kbps). Consequently, for a fixed spreading factor, the amount of processing in the Node B can vary significantly (for example in a ratio exceeding 3:1). This is not taken into account in the above prior art solution.        The number of Rake receiver fingers required for transmission channel or data estimation is highly dependent on the number of radio links. In the prior art solution, Node B algorithms such as load control or call admission control algorithms cannot take account of the maximum number of Rake receiver fingers because this kind of limitation is not related to the spreading factor.        The processing capacity signaled by the Node B to the CRNC is a global processing capacity that cannot take account of possible limitations on the processing capacity of the Node B.        
In the above first prior art patent application, a different approach is proposed whereby, to take account of possible limitations in the processing capacity of a Node B, the Node B signals to the CRNC one or more parameters such as the maximum number of radio links that can be set up and the maximum net bit rate for the radio links that have been set up, possibly for each transmission direction and/or for each type of channel coding that can be used.
A second prior art patent application (French Patent Application No. 01/00440 filed Jan. 12, 2001 by the applicant) proposes a different approach, whereby the global processing capacity (capacity credit) concept is retained, but the allocation cost is no longer signaled for each available value of the spreading factor, but instead for available bit rate values (the applicant has found that the bit rate is more representative of the processing capacity of a Node B than the spreading factor, as already indicated above).
Accordingly, the second prior art patent application discloses a method of managing processing resources in a mobile radio system in which a first entity manages radio resources and corresponding processing resources provided in a separate second entity, in which method:                the second entity signals to the first entity its global processing capacity (capacity credit) and the quantity of that global processing capacity (allocation cost) necessary for allocating radio resources for various bit rate values, and        on each allocation of radio resources, the first entity updates the capacity credit as a function of the corresponding bit rate.        
The expression “allocation of radio resources” is intended to include all operations likely to modify the allocation of radio resources within the system, including not only allocation operations as such but also de-allocation and reconfiguration operations.
In the UMTS, these operations correspond:                in the case of dedicated transport channels, to radio link set-up, radio link addition, radio link deletion and radio link reconfiguration procedures defined in 3GPP document 3G TS 25.433, and        in the case of common transport channels, to common transport channel set-up, common transport channel deletion, and common transport channel reconfiguration procedures defined in 3GPP document 3G TS 25.433.        
The expression “updating the capacity credit” is intended to include not only operations whereby the capacity credit is debited if new radio resources are required but also operations by which the capacity credit is credited if new radio resources are no longer necessary and are therefore returned.
Accordingly:                the capacity credit is debited for the radio link set-up, radio link addition and common transport channel set-up procedures,        the capacity credit is credited for the radio link deletion and common transport channel deletion procedures, and        the capacity credit is debited or credited for the radio link reconfiguration and common transport channel reconfiguration procedures, depending on whether the difference between the allocation cost for the new bit rate and that for the old bit rate is negative or positive.        
The second prior art patent application also proposes solutions to the following problems.
A first problem is that, although the number of spreading factors available is finite (in the UMTS, for example, eight spreading factor values are available: 4, 8, 16, 32, 64, 128, 256, 512), the bit rate can take any positive value. Clearly, it is not possible in practice or realistic for the Node B to signal to the CRNC the allocation cost for all bit rate values.
A second problem is that the CRNC does not have the bit rate available for updating the capacity credit on each allocation of resources as a function of the corresponding bit rate, at least in the present version of the standard. To the contrary, in the first prior art solution previously referred to, the CRNC knows the spreading factor because the SRNC signals the spreading factor to the CRNC when a new radio link is added, removed or reconfigured.
A third problem is that the bit rate may vary instead of being fixed. The spreading factor is fixed, however, at least for the downlink direction (it is also signaled, as previously indicated). The spreading factor for the uplink direction is also variable, but the applicant has found that this aspect of the problem is not taken into consideration in the prior art solution previously referred to using the spreading factor.
To solve the first problem, the second prior art patent application proposes signaling the cost for only a few typical values of the bit rate, which are referred to hereinafter as the reference bit rates, and further proposes a solution for determining the cost for any bit rate value from costs signaled for the reference bit rates. For example, linear interpolation can be used, which is the simplest solution, whilst ensuring that the cost always remains positive (i.e. if the result of the interpolation is negative, the resultant cost is zero).
For example, when the bit rate R is not a reference bit rate, the Consumption_cost is calculated as a function of costs Cinf and Csup corresponding to the reference bit rates Rinf and Rsup (with Rinf<Rsup) that are closest to the bit rate R, using the following equation:
                    Consumption_cost        =                              C            inf                    +                                                    R                -                                  R                  inf                                                                              R                  sup                                +                                  R                  inf                                                      ⁢                          (                                                C                  sup                                -                                  C                  inf                                            )                                                          (        1        )            
If the result is negative, the allocation cost can be set to zero, i.e.:                Consumption_cost=0.        
Other interpolation techniques can of course be used.
Examples of reference bit rates are 4.75 kbps, 12.2 kbps, 64 kbps, 144 kbps, 384 kbps, and 2 048 kbps.
To solve the second and third problems in the UMTS, for example, the second prior art patent application proposes deriving the bit rate as a function of a Transport Format Combination Set (TFCS) parameter.
A feature of the UMTS is the facility to transport several services on the same connection, in other words to have several transport channels (TrCH) on the same physical channel. Transport channels are processed separately in accordance with a channel coding scheme (including error detector coding, error corrector coding, bit rate adaptation and interleaving, as described with reference to FIG. 2), before being time-division multiplexed to form a coded composite transport channel (CCTrCH) to be transmitted on one or more physical channels. More information on these aspects of the UMTS can be found in the 3GPP document 3G TS25 212 V3.0.0.
Another feature of the UMTS is authorizing variable bit rates for users during calls. The data transported by the transport channels is organized into data units called transport blocks received with a period called the transmission time interval (TTI). The number and size of the transport blocks received for a given transport channel vary as a function of the bit rate. The transport format is defined as the known number and size of the transport blocks (and therefore the instantaneous bit rates) for a given transport channel. The transport format combination (TFC) is defined as a combination of transport formats authorized for different transport channels to be multiplexed onto the same coded composite transport channel. Finally, the transport format combination set (TFCS) is defined as the set of available combinations of transport formats. For more information on these aspects of the UMTS see the 3GPP document TS 25.302 V.3.7.0.
The bit rate for each TFC within a TFCS can therefore be calculated from the following equation:
                              br          j                =                              ∑                          k              =              1                        n                    ⁢                                                    N                k                                  (                  j                  )                                            ⁢                              L                k                                  (                  j                  )                                                                    TTI              k                                                          (        2        )                            in which brj is the bit rate of the jth TFC in the TFCS, n is the number of transport channels in the CCTrCH, Nk(j) and Lk(j) are respectively the number of transport blocks and the size (in bits) of the transport blocks for the kth transport channel in the jth TFC, and TTIk is the transmission time interval (TTI) of the kth transport channel (in seconds).        
Other formulas can of course be used, depending on how the bit rate for the data to be processed is defined.
Also, the problem is that the bit rate is not fixed, but can vary (i.e. any TFC within the TFCS can be used during the call), and this variation is not known (and cannot be known) a priori to the Node B or the UE. The simplest solution is to consider only the maximum bit rate or the bit rate that maximizes the allocation cost of all the TFC within the TFCS (this latter bit rate is usually equal to the maximum bit rate, but this is not always the case). When a new radio link is accepted, it is necessary to verify that the Node B has sufficient resources to process bit rates up to the maximum bit rate authorized for the new radio link.
If Maximum_bit_rate is the bit rate defined by the equation:Maximum_bit_rate=Maxjbrj  (3)                equation (1) then becomes:        
                    Consumption_cost        =                              C            inf                    +                                                                      Maximum_bit                  ⁢                  _rate                                -                                  R                  inf                                                                              R                  sup                                +                                  R                  inf                                                      ⁢                          (                                                C                  sup                                -                                  C                  inf                                            )                                                          (        4        )            
Processing resource management as described in the second prior art application is therefore based on the following parameters, which the Node B has to signal the to the CRNC for dedicated transport channels (which correspond to radio resources allocated individually to users) and for common transport channels (which correspond to radio resources shared between several users):                the total capacity (total credit) that defines the processing resources of the Node B, and        the consumption law that gives the allocation cost for the typical reference bit rates and enables the allocation cost to be determined for all the available bit rates (using interpolation techniques).        
As previously indicated, it would therefore be more advantageous to use a resource model based on the bit rate than to use a resource model based on the spreading factor. However, not all current equipment using a resource model based on the spreading factor can necessarily be modified to use a resource model based on the bit rate. Furthermore, in future equipment, using a resource model based on the spreading factor in addition to a resource model based on the bit rate may eventually be supported. A problem of backward compatibility could then arise between different types of equipment, depending on the type of resource model that they use.